Alkali metal fluorozincate and method for producing it

ABSTRACT

Novel methods for producing alkali metal fluorozincates, especially potassium fluorozincate. Products having defined particle size ranges are obtained depending on the sequence of introduction of the reactants alkali metal hydroxide, zinc oxide and hydrogen fluoride. The resulting alkali metal fluorozincates are particularly suitable for use as a fluxing agent or fluxing agent additive during brazing of aluminum or aluminum alloys and can be applied by wet or dry fluxing methods depending on the particle size of the alkali metal fluorozincate particles.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of international patentapplication no. PCT/EP01/03509, filed Mar. 28, 2001, designating theUnited States of America and published in German as WO 01/74715, theentire disclosure of which is incorporated herein by reference. Priorityis claimed based on Federal Republic of Germany patent application no.DE 100 16 257.6, filed Apr. 3, 2000.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method for producing alkalimetal fluorozincate, to the resulting product, and to the use thereof inbrazing aluminum.

[0003] Alkali metal fluorozincates, such as cesium fluorozincate andparticularly potassium fluorozincate, may be used as fluxing agents forbrazing aluminum components and components made of aluminum alloys. Inthis case, these compounds act not only as fluxing agents, but ratherthey produce a coating of the surface, in that zinc is deposited on thesurface of the components. The fluorozincates may also be used togetherwith alkali fluoroaluminates, for example potassium fluoroaluminateand/or cesium fluoroaluminate. These compounds also act as fluxingagents for brazing aluminum. German Published Application 199 13 111 A1discloses these types of fluxing agents and their use. The production ofalkali metal fluorozincates according to this published application isachieved by melting together alkali metal fluoride and zinc fluoride, byreacting alkali metal fluoride and zinc fluoride in aqueous phase, or byreacting zinc oxide with adducts of hydrogen fluoride and alkali metalfluoride.

SUMMARY OF THE INVENTION

[0004] It is an object of the invention to provide a method forproducing alkali metal fluorozincates which is capable of producingproducts having defined grain distribution ranges.

[0005] Another object of the invention is to provide an alkali metalfluorozincate which is especially suited for use as a fluxing agent orfluxing agent component.

[0006] These and other objects are achieved in accordance with thepresent invention by providing a method for producing alkali metalfluorozincate from alkali metal hydroxide, zinc oxide, and alkali metalfluoride or hydrogen fluoride in aqueous phase, wherein:

[0007] a) to produce fine alkali metal fluorozincate, alkali metalhydroxide and zinc oxide are mixed into a suspension and hydrogenfluoride is added, or

[0008] b) to produce medium fine alkali metal fluorozincate, hydrogenfluoride and zinc oxide are mixed with one another and alkali metalhydroxide is added, or

[0009] c) to produce coarse alkali metal fluorozincate, hydrogenfluoride and zinc oxide are mixed with one another and alkali metalfluoride is added.

[0010] In accordance with a further aspect of the invention, the objectsare achieved by providing a fine alkali metal fluorozincate produced bythe method of claim 1, and having a grain spectrum in which 50% of allparticles have a diameter <5 μm, or a medium fine alkali metalfluorozincate produced by the method of claim 1, and having a grainspectrum in which 50% of all particles have a diameter <11 μm, or acoarse alkali metal fluorozincate produced by the method of claim 1, andhaving a grain spectrum in which 50% of all particles have adiameter >11 μm.

[0011] In accordance with yet another aspect of the invention, theobjects are achieved by providing a method of fluxing an aluminum oraluminum alloy component for brazing, said method comprising applying tosaid component a fluxing agent comprising an alkali metal fluorozincateproduced as described above.

[0012] The present invention is based on the recognition that alkalimetal fluorozincate having specific grain size properties may beproduced as a function of the type of starting compounds and thesequence in which the starting reactants are brought into contact withone another.

[0013] The method according to the present invention for producingalkali metal fluorozincate from alkali metal hydroxide, zinc oxide, andalkali metal fluoride or hydrogen fluoride in aqueous phase isdistinguished in that

[0014] a) to produce fine alkali metal fluorozincate, alkali metalhydroxide and zinc oxide are mixed into a suspension, and hydrogenfluoride is added, or

[0015] b) to produce medium fine alkali metal fluorozincate, hydrogenfluoride and zinc oxide are mixed with one another, and potassiumhydroxide is added, or

[0016] c) to produce coarse alkali metal fluorozincate, zinc oxide andhydrogen fluoride are mixed with one another, and alkali metal fluorideis added.

[0017] Instead of zinc oxide, other zinc compounds, e.g., zinchydroxide, zinc carbonate, or zinc chloride, may be used. However, zincoxide is preferred.

[0018] In the context of the present invention, the term “fine” refersto an alkali metal fluorozincate in which 50% of the particles have aparticle size smaller than 5 μm and 90% of the particles have a particlesize of less than 9 μm; the remaining particles are then the same sizeor larger than the indicated values. Fine alkali metal fluorozincate ispreferably produced in which 50% of all particles have a diameter of<3.8 μm and 90% of all particles have a diameter of <8 μm. The size isdetermined using laser diffraction.

[0019] Within the context of the present invention, the term “mediumfine” refers to a product in which 50% of the particles of the materialproduced have a grain size of less than 11 μm and 90% have a grain sizeof less than 27 μm. The diameters of the remaining particles are at orabove the listed values. “Medium fine” preferably means that 50% of allparticles produced have a grain size of less than 10 μm and 90% of allparticles have a grain size of less than 26 μm. The particle sizeanalysis is again carried out by laser diffraction.

[0020] In the context of the present invention, the term “coarse” meansthat 50% of all particles have a grain size of less than 22 μm and 90%have a grain size of less than 40 μm. The term “coarse” preferably meansthat 50% of all particles have a grain size of less than 21 μm and 90%have a grain size of less than 39 μm. The diameters of the remainingparticles correspond to the recited dimensions or lie below them.

[0021] Alkali metal hydroxide, hydrogen fluoride, and alkali metalfluoride advantageously are used in the form of an aqueous solution. Thepreferred alkali metal is potassium.

[0022] In the first variant of the method according to the presentinvention, fine alkali metal fluorozincate is formed. The fine alkalimetal fluorozincate preferably is then isolated and dried. Surprisingly,the resulting product is finer than the product which results accordingto the method described in German Published Application 199 13 111. Forexample, alkali metal fluorozincate having a grain spectrum in which 50%of all particles have a diameter of <5 μm (measured by laserdiffraction) may be produced in this way.

[0023] Preferably, the procedure starts with potassium hydroxide as analkali hydroxide and produces potassium fluorozincate. In this case, thepotassium hydroxide is preferably used in the form of an aqueouspotassium hydroxide solution. The concentration of KOH in this potassiumhydroxide solution is advantageously in the range from 10 to 50weight-percent. KZnF₃ is particularly preferably produced. For thispurpose, potassium hydroxide, zinc oxide, and hydrogen fluoride arepreferably used in quantities such that the atomic ratio of K:Zn is inthe range from 1:1±0.05 and the atomic ratio of (K+Zn):F is in the rangefrom 1:3±0.05. The zinc oxide may also be replaced by a zinc oxideprecursor, for example, ZnCO₃ or ZnCl₂.

[0024] A suspension is formed, which then reacts further with hydrogenfluoride to obtain the desired alkali metal fluorozincate. Preferably, ahydrofluoric acid solution is used containing 10 to 99 weight-percentHF, preferably 20 to 40 weight-percent HF.

[0025] The suspension of alkali metal hydroxide and zinc oxide ispreferably produced at a temperature in the range from 15 to 85° C.,particularly at ambient temperature (approximately 20° C.). Theresulting intermediate product is advantageously reacted with hydrogenfluoride at a temperature in the range from approximately 20° C. to 90°C.

[0026] In the second variant of the present invention, medium finealkali metal fluorozincate is produced. Hydrogen fluoride and alkalihydroxide are preferably used as an aqueous solution. The preferredconcentrations correspond to those of the first variant. Alkalipreferably is represented by potassium. Hydrogen fluoride and zincoxide, which may be used in precursor form as zinc carbonate, forexample, are reacted at a temperature in the range from 20° C. (ambienttemperature) to 95° C. The subsequent reaction with the hydroxide ispreferably carried out at a temperature in the range from 70° C. to 90°C.

[0027] For the third variant, in which coarse alkali metal fluorozincateis produced, hydrogen fluoride is initially reacted with zinc oxide, andthen alkali metal fluoride is added. In this case as well, alkalipreferably is represented by potassium. Hydrogen fluoride is preferablyused in aqueous solution, the preferred concentration being as in thefirst variant. The alkali metal fluoride is also preferably used inaqueous solution. The preferred alkali metal fluoride concentration liesin the range from 25 to 40 weight-percent, particularly 28 to 32weight-percent. The temperature of the first reaction step is in thesame range as in variant 1 and variant 2. The addition of alkali metalfluoride is then preferably carried out at a temperature of 70° C. to90° C.

[0028] As noted above, the invention also relates to the fine, mediumfine, and/or coarse alkali metal fluorozincate, obtained according tothe variants of the method according to the present invention, havingthe further parameters indicated above in regard to the grain size.Potassium fluorozincate is preferred, particularly potassiumfluorozincate of the formula KZnF₃. In this case, the grain sizes of thefine product—in relation to 50% of the particles, d_(X50)—are in therange from 3 to 5 μm, those of the medium fine product are in the rangefrom 6 to 11 μm, and those of the coarse product are in the range from12 to 25 μm. Experiments have shown that it is possible to producepotassium fluorozincate, for example, having an even finer grainspectrum (e.g., 50% of the particles having a diameter of less than 3.3μm).

[0029] If a mixture of potassium hydroxide and rubidium, lithium, orcesium hydroxide is used, corresponding mixed fluorozincates of the typealkali_(1−x)alkali′_(x)ZnF₃ (x<1 and alkali not equal to alkali′) may beproduced. These mixed fluorozincates are also a subject of the presentinvention. In the case of mixed fluorozincates, potassium-cesiumfluorozincate (the ratio of potassium to cesium being arbitrary) ispreferred.

[0030] The fluorozincates according to the present invention areparticularly suitable for use as a fluxing agent for brazing aluminumand aluminum alloys. They are then used in a known way, for example, asdisclosed in German Published Application 199 13 111. The fine productis particularly suitable for application through wet fluxing, the coarseproduct for application by dry fluxing. The medium fine product is wellsuited for both purposes. Of course, mixtures having predeterminedproperties may also be produced.

[0031] The fluorozincates of the invention are also suitable for use asa solder fluxing agent additive for other fluxing agents, particularlyas a fluxing agent additive for potassium fluoroaluminate and cesiumfluoroaluminate.

[0032] The alkali metal fluorozincate according to the present inventionmay, of course, also be mixed with alkali metal fluorozincate producedin typical ways, in order to influence properties such as suspensibilityor fluidizability.

[0033] For example, the alkali metal fluorozincate of the invention maybe used mixed with solder metal, or with a precursor for a solder metal,for example with 5-95 weight-percent, relative to the total fluxingagent, of alkali metal fluorosilicate such as potassiumhexafluorosilicate, see European Patent Application 810 057 and GermanPublished Application 196 36 897, or with 10 to 80 weight-percent ofcopper, zinc, or germanium, which form a eutectic mixture with aluminum,see U.S. Pat. No. 5,190,596.

[0034] The alkali metal fluorozincate of the invention may also be usedin a mixture with alkali fluoroaluminates, for example, with KAlF₄ orK₂AlF₅, as is analogously described in German Published Application 19913 111, or with cesium fluoroaluminate. The latter is advantageous forAl alloys having larger proportions of Mg.

[0035] Further auxiliary agents may be included, for example, asdescribed in German Published Application 199 13 111, p. 3. For example,binders or dispersing agents may be included.

[0036] The fluxing agent may be applied in a known way to the componentsmade of aluminum or aluminum alloys to be bonded. Dry application on thebasis of electrostatic spraying technology is possible due to the goodfluidizing properties of the fluxing agent. Alternatively, the fluxingagent may be applied in the form of aqueous and/or organic suspensionsor as a paste to the materials to be bonded. Aqueous or organic slurriesadvantageously contain 15 to 75 weight-percent of the fluxing agent.Suspensions of the fluxing agent in organic liquids, expediently thesubstances typically used as organic solvents, such as alcohols,particularly methanol, ethanol, propanol, or isopropanol, as well aspolyols, may also be used. Other organic liquids (“carriers”) includeethers, for example, diethylene glycol monobutyl ether, ketones such asacetone, and esters of alcohols, diols, or polyols. An example of abinder for application as a paste is ethyl cellulose. Using filmformers, typically polymers which are soluble in organic solvents, e.g.,acetone, fluxing agent may be applied, possibly with brazing metal orsolder precursors, onto the workpiece and produce a strongly adheringfilm after evaporation of the solvent. Suitable polymers include, forexample, (meth)acrylates. During brazing, the film former vaporizes.

[0037] During application, the brazing metal, if it is necessary, may becontained in the fluxing agent (as an admixed powder), it may already beapplied as a plating on the components to be brazed, or it may beapplied in addition to the fluxing agent.

[0038] The brazing temperature is a function of the brazing materialused or the metal or material forming the brazing material. Below asolder metal liquidization temperature of 450° C., according to thedefinition, one refers to soft soldering (=“soldering”), above this, tohard soldering (=“brazing”). There are low melting solders, such aszinc-aluminum solders, which may be used from 390° C., or pure zincsolder, which may be used from 420° C. for soldering. Other solders maybe brazed at higher temperatures. Al—Si—[Cu] solders may be used from[530° C.] or 577° C., respectively.

[0039] In general, a brazing temperature of 600° C. is sufficient.Preferably, soldering or brazing is performed from 390° C. to 600° C.,particularly from 420 to 590° C. Ambient pressure is used in this case.It is also possible to braze, for example, in vacuum, with vaporizationof the fluxing agent, as described in Japanese Patent Application03/099795, is also possible. Flame or furnace soldering may also becarried out, particularly in inert atmosphere (e.g., N₂ atmosphere).

EXAMPLE 1 Fine Product

[0040] Reaction: KOH+ZnO+3HF->KZnF₃+2H₂O Batch: 1) 0.5 ml KOH solution45.2% w/w 62.3 g ZnO (99.9 cm Aldrich) 40.9 g completely desalinatedwater 30.6 ml (CD water) 2) HF solution 49.6% w/w 60.6 g CD water 60.6ml cooling CD water   33 ml

[0041] Procedure:

[0042] 62.3 g of the 45.2% KOH w/w was diluted with 30.6 ml of CD waterand subsequently 40.9 g of ZnO was added and slurried while beingstirred. An HF solution made of 60.6 g of the 49.6% HF solution and 60.6g of CD water was then introduced from a dropping funnel within 90seconds. The temperature rose from 24.6° C. to 85.9° C. For cooling, 33ml of CD water was added. The mixture was then stirred for one hour at80° C., the precipitate was filtered off and dried for four hours at180° C. Yield: quantitative XRD = KZnF₃ Grain size (laser diffraction):X₅₀ = 3.28 μm X₉₀ = 6.98 μm

EXAMPLE 2 Medium Fine Product

[0043] The product was produced as described in Example 1 with theexception that batch 1 was produced by reacting HF solution with zincoxide, and batch 2 was the KOH solution. Grain size X₅₀ = 9.47 μm X₉₀ =25.75 μm

EXAMPLE 3 Coarse Product

[0044] The produce was produced as described in Example 1 with theexception that batch 1 was produced by reaction of HF solution with zincoxide, and batch 2 was a solution of KF in water. Grain size X₅₀ = 20.50μm Grain size X₉₀ = 38.18 μm

EXAMPLE 4 Use for Brazing

[0045] A triturate of 3 g of the substance from Example 1 withisopropanol was applied to a 25×25 mm aluminum sheet, which was platedwith AlSi solder, and an aluminum angle was placed on top. This assemblywas thermally treated in a laboratory brazing oven in accordance withthe Nocolok® brazing process conditions. The brazing of the assembly was100%.

[0046] The foregoing description and examples have been set forth merelyto illustrate the invention and are not intended to be limiting. Sincemodifications of the described embodiments incorporating the spirit andsubstance of the invention may occur to persons skilled in the art, theinvention should be construed broadly to include all variations withinthe scope of the appended claims and equivalents thereof.

What is claimed is:
 1. A method for producing alkali metal fluorozincatefrom alkali metal hydroxide, zinc oxide, and alkali metal fluoride orhydrogen fluoride in aqueous phase, wherein: a) to produce fine alkalimetal fluorozincate, alkali metal hydroxide and zinc oxide are mixedinto a suspension and hydrogen fluoride is added, or b) to producemedium fine alkali metal fluorozincate, hydrogen fluoride and zinc oxideare mixed with one another and alkali metal hydroxide is added, or c) toproduce coarse alkali metal fluorozincate, hydrogen fluoride and zincoxide are mixed with one another and alkali metal fluoride is added. 2.A method according to claim 1, for producing fine alkali metalfluorozincates, wherein alkali metal hydroxide is mixed into asuspension with zinc oxide in aqueous phase, and the suspension isreacted with hydrogen fluoride to form fine alkali metal fluorozincate.3. A method according to claim 1, wherein the alkali metal hydroxide ispotassium hydroxide and potassium fluorozincate is produced.
 4. A methodaccording to claim 1, wherein potassium hydroxide is used in the form ofas an aqueous potassium hydroxide solution.
 5. A method according toclaim 3, wherein the atomic ratio of K:Zn is in the range from 1:1±0.05,and the atomic ratio of (K+Zn):F is in the range from 1:3±0.05.
 6. Amethod according to claim 1, further comprising isolating and drying thealkali metal fluorozincate produced.
 7. A method according to claim 2,wherein the suspension of alkali metal hydroxide and zinc oxide isproduced at a temperature in the range from 15 to 85° C., and thesuspension subsequently is reacted with hydrogen fluoride at atemperature up to 90° C.
 8. A fine alkali metal fluorozincate producedby the method of claim 1, and having a grain spectrum in which 50% ofall particles have a diameter <5 μm.
 9. An alkali metal fluorozincateaccording to claim 8, wherein alkali metal is potassium.
 10. A potassiumfluorozincate according to claim 9, having a grain spectrum in which 50%of all particles have a diameter <3.8 μm.
 11. A medium fine alkali metalfluorozincate produced by the method of claim 1, and having a grainspectrum in which 50% of all particles have a diameter <11 μm.
 12. Acoarse alkali metal fluorozincate produced by the method of claim 1, andhaving a grain spectrum in which 50% of all particles have adiameter >11 μm.
 13. A method of fluxing an aluminum or aluminum alloycomponent for brazing, said method comprising applying to said componenta fluxing agent comprising an alkali metal fluorozincate according claim8.
 14. A method according to claim 13, wherein said fluxing agent isapplied by electrostatic dry fluxing.
 15. A method of fluxing analuminum or aluminum alloy component to be brazed, said methodcomprising applying to said component a fluxing agent comprising analkali metal fluorozincate according to claim
 11. 16. A method offluxing an aluminum or aluminum alloy component to be brazed, saidmethod comprising applying to said component a fluxing agent comprisingan alkali metal fluorozincate according to claim
 12. 17. A methodaccording to claim 16, wherein said fluxing agent is applied by wetfluxing from an aqueous or organic suspension.